Intercomparison of 15 aerodynamic particle size spectrometers (APS 3321): uncertainties in particle sizing and number size distribution

摘要

Aerodynamic particle size spectrometers are a well-established method tomeasure number size distributions of coarse mode particles in theatmosphere. Quality assurance is essential for atmospheric observationalaerosol networks to obtain comparable results with known uncertainties. In alaboratory study within the framework of ACTRIS (Aerosols, Clouds, and Tracegases Research Infrastructure Network), 15 aerodynamic particle sizespectrometers (APS model 3321, TSI Inc., St. Paul, MN, USA) were comparedwith a focus on flow rates, particle sizing, and the unit-to-unit variability ofthe particle number size distribution.Flow rate deviations were relatively small (within a few percent), while thesizing accuracy was found to be within 10 % compared to polystyrene latex(PSL) reference particles. The unit-to-unit variability in terms of theparticle number size distribution during this study was within 10 % to20 % for particles in the range of 0.9 up to 3 µm, which isacceptable for atmospheric measurements. For particles smaller than that,the variability increased up to 60 %, probably caused by differences inthe counting efficiencies of individual units. Number size distribution datafor particles smaller than 0.9 µm in aerodynamic diameter should only be used with caution. For particles larger than 3 µm, theunit-to-unit variability increased as well. A possible reason is aninsufficient sizing accuracy in combination with a steeply sloping particlenumber size distribution and the increasing uncertainty due to decreasingcounting. Particularly this uncertainty of the particle number size distribution must be considered if higher moments of the size distribution suchas the particle volume or mass are calculated, which require the conversionof the aerodynamic diameter measured to a volume equivalent diameter.In order to perform a quantitative quality assurance, a traceable referencemethod for the particle number concentration in the size range 0.5–3 µm is needed.